In stand mixer or with hand mixer cream butter with white and brown sugar. Add egg and mix to combine.
Sift together flour, baking soda, baking powder and salt.
Add peanut butter to sugar mixture and combine. Add flour mixture all at once and mix only until incorporated. Do not over mix.
Spread evenly in a 9 inch square pan. Bake at 350° for 25 minutes. Layer will be just starting to brown on the edges and slightly raised.
Meanwhile, chop chocolate into small pieces. Melt in top layer of double boiler or in a bowl over a container of very hot water. Stir until completely melted. Stir in peanut butter.
Chop nuts and pretzels. Pour most of chocolate mixture over top of peanut butter layer. Spread evenly to edges. Sprinkle nut and pretzel mixture over top. Press down gently to ensure toppings stick into the chocolate. Drizzle remaining chocolate over top.
Wait until chocolate layer has cooled enough to set before cutting into squares.